Method for Producing Silica Sol and Silica Sol

The present disclosure relates to a method for producing a silica sol and a silica sol.

Conventionally, chemical mechanical polishing (CMP) using a polishing composition has been performed on the surface of materials such as metals, semimetals, nonmetals, and their oxides. It is known that such polishing composition generally has a configuration in which an aqueous solution having a chemical polishing action and particles (abrasive grains) having a mechanical polishing action are mixed and dispersed, and a silica sol is used as the abrasive grains. In such a case, by heteromorphizing silica particles, high friction can be obtained during polishing, and the polishing rate can be further improved.

JP 2018-168031 A discloses, as a method for heteromorphizing silica particles, a method for producing a silica sol, including mixing a liquid (A) containing an alkaline catalyst, water, a first organic solvent, and silica particles for association with a liquid (B) containing at least one of tetramethoxysilane and a condensate thereof and a second organic solvent to prepare a reaction liquid, in which during the mixing, an addition rate of the liquid (B) is 8.5×10to 5.6×10mol/min in terms of silicon atoms with respect to 1 mol of the water contained in the liquid (A).

In the technique described in JP 2018-168031 A, a silica sol including highly associated silica particles can be obtained. However, for example, when it is intended to eliminate the surface roughness of a wafer, it is required to increase the ratio of heteromorphized silica particles included in the silica sol.

Therefore, the present disclosure has been made in view of the above circumstances, and an object thereof is to provide a method for producing a silica sol capable of increasing the ratio of heteromorphized silica particles.

The present inventors have conducted intensive studies in view of the above circumstances. As a result, the present inventors have found that the above effect can be obtained by a method for producing a silica sol, including preparing a first liquid including silica core particles having an average value of longest diameters of primary particles of 20 nm or less, and preparing a second liquid including linked silica core particles by holding the first liquid for 72 hours or longer, thereby completing the present disclosure.

Hereinafter, embodiments according to an aspect of the present disclosure will be described. The present disclosure is not limited only to the following embodiments, and various modifications can be made within the scope of claims. The embodiments described in the present specification may be other embodiments by being arbitrarily combined.

In the present specification, the phrase “X to Y” indicating a range means “X or more and Y or less”. Unless otherwise specified, operations and measurements of physical properties and the like are measured under the conditions of room temperature (20 to 25° C.)/relative humidity of 40 to 50% RH.

An aspect of the present disclosure relates to a method for producing a silica sol, including preparing a first liquid including silica core particles having an average value of longest diameters of primary particles of 20 nm or less, and preparing a second liquid including linked silica core particles by holding the first liquid for 72 hours or longer. With such a configuration, a silica sol including a large amount of heteromorphized silica particles is provided. According to an aspect of the present disclosure, there is provided a method for producing a silica sol capable of increasing the ratio of heteromorphized silica particles.

In the present disclosure, the silica sol including heteromorphized silica particles means that the silica particles in the silica sol satisfy any one of an average aspect ratio of 1.5 or more and an average circularity of 0.80 or less. The silica particles in the silica sol according to the present aspect preferably satisfy an average aspect ratio of 1.5 or more and an average circularity of 0.80 or less, and more preferably satisfy an average aspect ratio of 1.5 or more, an average circularity of 0.80 or less, and an average association degree of 1.7 or more. The average association degree, the average aspect ratio, and the average circularity will be described in the silica sol described later.

The reason why the above effect is achieved by the production method according to the present aspect is not necessarily clear, but it is considered as follows.

In the method for producing a silica sol according to the present aspect, it is considered that the surface of the silica core particles included in the first liquid is activated because the amount of hydroxyl groups is large. Therefore, the present inventors have considered that it is possible to obtain silica particles in which silica core particles are bonded to each other to be heteromorphized by a condensation reaction between hydroxyl groups on the surfaces of silica core particles and hydroxyl groups on the surfaces of other silica particles. As a result of studies by the present inventors, it has been found that the silica core particles may be bonded to each other by holding the first liquid for a long time (72 hours or longer). The present inventors have found that when the average value of longest diameters of primary particles of silica core particles exceeds a certain value (20 nm), heteromorphized silica particles cannot be obtained even if the first liquid is held for a long time. Based on these findings, the present inventors have completed the present disclosure.

The above mechanism is based on speculation, and its correctness does not affect the technical scope of the present disclosure.

Hereinafter, constituent requirements of the method for producing a silica sol according to the present aspect will be described.

In the silica core particle preparation step, a first liquid including silica core particles having an average value of longest diameters of primary particles of 20 nm or less is prepared.

In the present specification, the silica core particles included in the first liquid include silica particles in which two or more primary particles are bonded.

In the present specification, the average value of longest diameters of primary particles of silica core particles means a value obtained by measuring the longest diameters of primary particles of each of silica core particles in a captured scanning electron microscope (SEM) image as exemplified by double-headed arrows in, and calculating the average value of longest diameters of primary particles of all the silica core particles in the SEM image. As the average value of longest diameters of primary particles, a value calculated by the method described in Examples is adopted.

In the present specification, the average value of longest diameters of primary particles of the silica core particles is also simply referred to as “the average longest diameter of the silica core particles”.

The first liquid according to the present aspect includes silica core particles having an average value of longest diameters of primary particles of 20 nm or less. When the average value of longest diameters of primary particles of the silica core particles exceeds 20 nm, heteromorphized silica particles cannot be obtained in the bonding step described later (see Comparative Example 4). The upper limit of the average longest diameter of the silica core particles is preferably 18 nm or less. The lower limit of the average longest diameter of the silica core particles is not particularly limited, and is, for example, 2 nm or more, preferably 5 nm or more, and more preferably 10 nm or more. The average longest diameter of the silica core particles is preferably 2 nm or more and 20 nm or less, more preferably 5 nm or more and 20 nm or less, still more preferably 10 nm or more and 20 nm or less, and particularly preferably 10 nm or more and 18 nm or less.

In the silica core particle preparation step, the method for preparing the first liquid is not particularly limited, and a conventionally known method can be used. For example, alkoxysilane or a condensate thereof can be reacted in an organic solvent containing water and an alkaline catalyst to obtain a first liquid including silica core particles.

Hereinafter, an embodiment of the silica core particle preparation step will be described.

In an embodiment, the silica core particle preparation step includes adding and mixing a liquid (B) containing at least one of alkoxysilane and a condensate thereof and a second organic solvent or the liquid (B) and a liquid (C) containing water and being free of an alkaline catalyst in a liquid (A) containing an alkaline catalyst, water, and a first organic solvent and terminating the addition to prepare the first liquid when the average value of longest diameters of primary particles of the silica core particles is 20 nm or less.

The liquid (A) according to the present embodiment contains an alkaline catalyst, water, and a first organic solvent. The liquid (A) can contain other components in addition to the alkaline catalyst, water, and the first organic solvent as long as the effects of the present disclosure are not impaired.

In a preferred embodiment, the liquid (A) consists of an alkaline catalyst, water, and a first organic solvent. When the liquid (A) consists of an alkaline catalyst, water, and a first organic solvent, impurities contained in the first liquid can be reduced as much as possible. As a result, when the silica sol obtained by the production method of the present disclosure is used for a polishing slurry, it is possible to suppress the influence of impurities on polishing. It can be used also for the use applications in an object to be polished in which metal impurities are disliked, such as silicon wafers and device wafers, and a polishing slurry which is applicable widely can be provided.

As the alkaline catalyst contained in the liquid (A), a conventionally known alkaline catalyst can be used. From the viewpoint that contamination of metal impurities and the like can be minimized, examples of the alkaline catalyst include ammonia, ammonium salts such as tetramethylammonium hydroxide, ethylene diamine, diethylene triamine, triethylene tetramine, urea, monoethanolamine, diethanolamine, triethanolamine, and tetramethylquanidine. Among these, from the viewpoint of excellent catalytic action, ammonia, and ammonium salts such as tetramethylammonium hydroxide are more preferable, and ammonia is still more preferable. Since ammonia has high volatility, ammonia can be easily removed in the process of producing a silica sol. Note that, the alkaline catalyst may be used singly or as a mixture of two or more kinds thereof. The alkaline catalyst may be in the form of an aqueous solution.

As the water contained in the liquid (A), pure water or ultrapure water is preferably used from the viewpoint of minimizing contamination of metal impurities and the like. When the alkaline catalyst is in the form of an aqueous solution, water to be contained therein is water contained in the liquid (A). Therefore, the water contained in the aqueous solution of the alkaline catalyst is also preferably pure water or ultrapure water.

As the first organic solvent contained in the liquid (A), a hydrophilic organic solvent is preferably used, and specific examples thereof include alcohols such as methanol, ethanol, n-propanol, isopropanol, ethylene glycol, propylene glycol, and 1,4-butanediol; and ketones such as acetone and methyl ethyl ketone. The first organic solvent may be used singly or as a mixture of two or more kinds thereof.

As the first organic solvent according to the present embodiment, alcohols are preferable. By using alcohols, there is an effect that alcohols and water can be easily substituted by heating distillation when a water substitution step described later is performed. From the viewpoint of recovery and reuse of the organic solvent, it is preferable to use the same kind of alcohol as the alcohol generated by hydrolysis of alkoxysilane.

Among the alcohols, at least one of methanol, ethanol, isopropanol, and the like is more preferable. When tetramethoxysilane is used as alkoxysilane, the first organic solvent is preferably methanol.

The contents of the alkaline catalyst, water, and the first organic solvent in the liquid (A) are not particularly limited, and can be appropriately adjusted in order to set the average longest diameter of the silica core particles to a desired value.

The lower limit of the content of the alkaline catalyst (for example, ammonia) in the liquid (A) is, for example, 0.1 mass % or more and preferably 0.3 mass % or more with respect to the total amount (100 mass %) of the liquid (A) from the viewpoint of the action as a hydrolysis catalyst or the growth of silica particles. The lower limit of the content of the alkaline catalyst (for example, ammonia) may be 0.5 mass % or more, 1.0 mass % or more, or 2.0 mass % or more with respect to the total amount (100 mass %) of the liquid (A). The upper limit of the content of the alkaline catalyst (for example, ammonia) is not particularly limited. The upper limit of the content of the alkaline catalyst (for example, ammonia) is preferably 50 mass % or less with respect to the total amount (100 mass %) of the liquid (A) from the viewpoint of productivity and cost. The upper limit of the content of the alkaline catalyst (for example, ammonia) may be 40 mass % or less, 20 mass % or less, 15 mass % or less, or 10 mass % or less with respect to the total amount (100 mass %) of the liquid (A). The content of the alkaline catalyst (for example, ammonia) may be 0.1 mass % or more and 50 mass % or less, 0.3 mass % or more and 40 mass % or less, 0.5 mass % or more and 20 mass % or less, 1.0 mass % or more and 15 mass % or less, or 2.0 mass % or more and 10 mass % or less with respect to the total amount (100 mass %) of the liquid (A).

The content of water in the liquid (A) is adjusted in accordance with the amount of alkoxysilane or a condensate thereof used in the reaction. The lower limit of the content of water is preferably 5 mass % or more with respect to the total amount (100 mass %) of the liquid (A) from the viewpoint of hydrolysis of alkoxysilane. The upper limit of the content of water is preferably 50 mass % or less and more preferably 40 mass % or less with respect to the total amount (100 mass %) of the liquid (A) from the viewpoint of compatibility with the liquid (B). The upper limit of the content of water may be 20 mass % or less with respect to the total amount (100 mass %) of the liquid (A). The content of water may be 5 mass % or more and 50 mass % or less, 5 mass % or more and 40 mass % or less, or 5 mass % or more and 20 mass % or less with respect to the total amount (100 mass %) of the liquid (A).

The lower limit of the content of the first organic solvent (for example, methanol) in the liquid (A) is preferably 10 mass % or more and more preferably 20 mass % or more with respect to the total amount (100 mass %) of the liquid (A) from the viewpoint of compatibility with the liquid (B). The lower limit of the content of the first organic solvent (for example, methanol) may be 50 mass % or more or 75 mass % or more with respect to the total amount (100 mass %) of the liquid (A). The upper limit of the content of the first organic solvent (for example, methanol) is preferably 98 mass % or less and more preferably 95 mass % or less with respect to the total amount (100 mass %) of the liquid (A) from the viewpoint of dispersibility. The upper limit of the content of the first organic solvent (for example, methanol) may be 90 mass % or less or 85 mass % or less with respect to the total amount (100 mass %) of the liquid (A). The content of the first organic solvent (for example, methanol) may be 10 mass % or more and 98 mass % or less, 20 mass % or more and 95 mass % or less, 50 mass % or more and 90 mass % or less, or 75 mass % or more and 85 mass % or less with respect to the total amount (100 mass %) of the liquid (A).

The method for producing the liquid (A) is not particularly limited, and for example, a method of stirring and mixing an alkaline catalyst, water, the first organic solvent, and other components as necessary can be used.

The liquid (B) according to the present embodiment contains at least one of alkoxysilane and a condensate thereof and a second organic solvent. The liquid (B) can contain other components in addition to at least one of alkoxysilane and a condensate thereof and the second organic solvent as long as the effects of the present disclosure are not impaired.

In the present specification, “at least one of alkoxysilane and a condensate thereof” is also collectively referred to simply as “alkoxysilane and the like”.

In a preferred embodiment, the liquid (B) consists of at least one of alkoxysilane and a condensate thereof and a second organic solvent. When the liquid (B) consists of at least one of alkoxysilane and a condensate thereof and a second organic solvent, impurities contained in the first liquid can be reduced as much as possible. As a result, when the silica sol obtained by the production method of the present disclosure is used for a polishing slurry, it is possible to suppress the influence of impurities on polishing. It can be used also for the use applications in an object to be polished in which metal impurities are disliked, such as silicon wafers and device wafers, and a polishing slurry which is applicable widely can be provided.

Examples of alkoxysilane or a condensate thereof contained in the liquid (B) include tetramethoxysilane, tetraethoxysilane, tetrapropoxysilane, and condensates thereof. These may be used singly or in combination of two or more kinds thereof. Among them, tetramethoxysilane is preferable from the viewpoint of having appropriate hydrolysis reactivity.

As the second organic solvent contained in the liquid (B), a hydrophilic organic solvent is preferably used, and specific examples thereof include alcohols such as methanol, ethanol, n-propanol, isopropanol, ethylene glycol, propylene glycol, and 1,4-butanediol; and ketones such as acetone and methyl ethyl ketone.

As the second organic solvent according to the present embodiment, alcohols are preferable. By using alcohols, there is an effect that alcohols and water can be easily substituted by heating distillation when a water substitution step described later is performed. From the viewpoint of recovery and reuse of the organic solvent, it is preferable to use the same kind of alcohol as the alcohol generated by hydrolysis of alkoxysilane.

Among the alcohols, at least one of methanol, ethanol, isopropanol, and the like is more preferable. When tetramethoxysilane is used as alkoxysilane, the second organic solvent is preferably methanol.

The contents of the alkoxysilane and the like and the second organic solvent in the liquid (B) are not particularly limited, and can be appropriately adjusted in order to set the average longest diameter of the silica core particles to a desired value.

The lower limit of the content of alkoxysilane and the like (for example, tetramethoxysilane and the like) in the liquid (B) is preferably 50 mass % or more, more preferably 60 mass % or more, still more preferably 70 mass % or more, and particularly preferably 75 mass % or more. The upper limit of the content of alkoxysilane and the like (for example, tetramethoxysilane and the like) is preferably 98 mass % or less, more preferably 95 mass % or less, still more preferably 90 mass % or less, and particularly preferably 85 mass % or less. The content of alkoxysilane and the like (for example, tetramethoxysilane and the like) is preferably 50 mass % or more and 98 mass % or less, more preferably 60 mass % or more and 95 mass % or less, still more preferably 70 mass % or more and 90 mass % or less, and particularly preferably 75 mass % or more and 85 mass % or less.

The lower limit of the content of the second organic solvent (for example, methanol) in the liquid (B) is preferably 2 mass % or more, more preferably 5 mass % or more, still more preferably 10 mass % or more, and particularly preferably 15 mass % or more. The upper limit of the content of the second organic solvent (for example, methanol) is preferably 50 mass % or less, more preferably 40 mass % or less, still more preferably 30 mass % or less, and particularly preferably 25 mass % or less. The content of the second organic solvent (for example, methanol) is preferably 2 mass % or more and 50 mass % or less, more preferably 5 mass % or more and 40 mass % or less, still more preferably 10 mass % or more and 30 mass % or less, and particularly preferably 15 mass % or more and 25 mass % or less.

When the contents of the alkoxysilane and the like and the second organic solvent in the liquid (B) are in the above ranges, miscibility when mixed with the liquid (A) can be improved.

The alkoxysilane condensate in the liquid (B) is, for example, a di- to dodecamer, and preferably a tetra- to octamer.

The method for producing the liquid (B) is not particularly limited. From the viewpoint of miscibility, the method for producing the liquid (B) preferably uses a method in which alkoxysilane and the like and, if necessary, other components are stirred and mixed in the second organic solvent.

The liquid (C) according to the present embodiment contains water and being free of an alkaline catalyst. Since the liquid (C) does not contain an alkaline catalyst, it is possible to suppress a local increase in the concentration of the alkaline catalyst at the time of mixing. The liquid (C) can contain other components in addition to the alkaline catalyst as long as the effects of the present disclosure are not impaired.

In a preferred embodiment, the liquid (C) consists of water. When the liquid (C) consists of water, impurities contained in the first liquid can be reduced as much as possible. As a result, when the silica sol obtained by the production method of the present disclosure is used for a polishing slurry, it is possible to suppress the influence of impurities on polishing. It can be used also for the use applications in an object to be polished in which metal impurities are disliked, such as silicon wafers and device wafers, and a polishing slurry which is applicable widely can be provided.

As the water contained in the liquid (C), pure water or ultrapure water is preferably used from the viewpoint of minimizing contamination of metal impurities and the like.